Activation control apparatus and method of air bag system

ABSTRACT

An activation control apparatus of an air bag system of a motor vehicle includes a first sensor disposed at a predetermined position within a vehicle body, for generating a signal indicative of an impact applied to the vehicle, and at least one second sensor disposed frontwardly of the position of the first sensor, for generating a signal indicative of an impact applied to the vehicle. The control apparatus is operable to activate an air bag device when a parameter determined based on the output signal of the first sensor exceeds a predetermined threshold pattern that is determined based on the output signal of the second sensor. When the predetermined threshold pattern is changed from a reference pattern to a desired threshold pattern that provides lower threshold values, the pattern is changed step by step at predetermined time intervals, without skipping an intermediate pattern or patterns between the reference pattern and the desired pattern.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The invention relates generally to activation control apparatusand method of an air bag system of a motor vehicle, and in particular tosuch an air-bag activation control apparatus that is arranged toactivate an air bag device in an appropriate situation so as to protectan occupant upon collision of the vehicle with an object.

[0003] 2. Description of Related Art

[0004] One type of an activation control apparatus of an air bag systemas disclosed in Japanese laid-open Patent Publication (Kokai) No.11-286257 has been known in the art. The activation control apparatus ofthis type includes a floor sensor that is disposed in a floor tunnel ofthe vehicle body and is adapted to generate a signal indicative of animpact applied to the vehicle floor portion at which the sensor islocated. When a parameter determined based on the output signal of thefloor sensor exceeds a threshold value, the activation control apparatusoperates to activate an air bag device so as to deploy or inflate an airbag. The apparatus further includes satellite sensors that are disposedin a front portion of the vehicle body and is adapted to generatesignals indicative of an impact that is applied to the vehicle frontportion in which the sensors are located. The above-indicated thresholdvalue is reduced by an amount that increases with an increase in theimpact that is applied to the front portion of the vehicle body and isdetermined based on the output signals of the satellite sensors. Withthe threshold value thus reduced, the air bag is made more likely todeploy as the magnitude of the impact applied to the front portion ofthe vehicle body increases. With the above-described known apparatus,therefore, the air bag device for protecting a vehicle occupant can beactivated at an appropriate time or in an appropriate situation.

[0005] In the known air-bag activation control apparatus as describedabove, the amount of reduction of the threshold value serving as acriterion for deployment of the air bag is increased with an increase inthe magnitude of an impact that is applied to the vehicle front portionand is determined based on the output signals of the satellite sensors.If the reduction of the threshold value to a desired value isaccomplished in one step with no regard to a difference between thethreshold value and the desired value, namely, with no regard to themagnitude of the impact applied to the vehicle front portion, thelikelihood that the air bag will deploy increases by a large degree at atime. This may happen even in the case where it is determined that agreat impact is applied to the vehicle front portion, actually becauseof an abnormality or defect in the satellite sensors, or the like. Insuch a case, the air bag may deploy by mistake. In order to cause theair bag to deploy at an appropriate time, therefore, it is consideredinappropriate or undesirable to reduce the threshold value to thedesired value in one step or at a time. In the known apparatus, however,no special measure has been taken for switching or reducing thethreshold value to a desired value without causing erroneous deploymentof the air bag.

SUMMARY OF THE INVENTION

[0006] It is therefore an object of the invention to provide anactivation control apparatus of an air bag system, which is able toactivate an air bag system at an appropriate time or in an appropriatesituation, by switching a threshold value as a criterion for deploymentof an air bag to a desired value step by step. It is another object ofthe invention to provide a method for controlling activation of an airbag device of an air bag system.

[0007] To accomplish the above object and/or other object(s), one aspectof the invention provides1 an activation control apparatus of an air bagsystem of a motor vehicle, which comprises: (a) a first sensor that isdisposed at a predetermined position within a vehicle body, and isoperable to generate an output signal indicative of an impact applied tothe vehicle, (b) activation control means for activating an air bagdevice when a parameter determined based on the output signal of thefirst sensor exceeds a predetermined threshold value, (c) a secondsensor that is disposed in the vehicle body frontwardly of thepredetermined position of the first sensor, and is operable to generatean output signal indicative of an impact applied to the vehicle, (d)threshold setting means for setting the predetermined threshold value toa first value selected from at least three candidate values, based onthe output signal of the second sensor, and (e) threshold switchingmeans for changing the predetermined threshold value from the firstvalue to a second value also selected from the at least three candidatevalues, by performing two or more switching operations to switch thepredetermined threshold value step by step at predetermined timeintervals when at least one of the at least three candidate values ispresent between the first value and the second value.

[0008] In the activation control apparatus constructed as describedabove, when the predetermine threshold value that has been set to acertain value is changed or updated to a desired value, thepredetermined threshold value is switched from the current value to thedesired value step by step at predetermined time intervals when at leastone of the three or more candidate values is present between the currentvalue and the desired value. In this case, the predetermined thresholdvalue is prevented from switching from the current value to the desiredvalue at a time while jumping an intermediate value(s). Therefore, evenif the predetermined threshold value greatly changes due to, forexample, an abnormality or a defect in the second sensor, the abovearrangement can avoid a situation that the air bag device is suddenlymade much easier to activate, or a situation that the air bag device issuddenly made much difficult to activate. Consequently, the air bagdevice is prevented from being activated by mistake. Thus, according tothe invention, the air bag device can be activated in an appropriatesituation.

[0009] According to another aspect of the invention, there is providedan activation control apparatus of an air bag system of a motor vehicle,which comprises: (a) a first sensor that is disposed at a predeterminedposition within a vehicle body, and is operable to generate an outputsignal indicative of an impact applied to the vehicle, (b) activationcontrol means for activating an air bag device when a parameterdetermined based on the output signal of the first sensor exceeds apredetermined threshold pattern, (c) a second sensor that is disposed inthe vehicle body frontwardly of the predetermined position of the firstsensor, and is operable to generate an output signal indicative of animpact applied to the vehicle, (d) threshold pattern setting means forsetting the predetermined threshold pattern to a first pattern selectedfrom at least three candidate patterns, based on the output signal ofthe second sensor, and (e) threshold pattern switching means forchanging the predetermined threshold pattern from the first pattern to asecond pattern also selected from the at least three candidate patterns,by performing two or more switching operations to switch thepredetermined threshold pattern step by step at predetermined timeintervals when at least one of the at least three candidate patterns ispresent between the first pattern and the second pattern.

[0010] In the activation control apparatus constructed as describedabove, when the predetermine threshold pattern that has been set to acertain pattern is changed or updated to a desired pattern, thepredetermined threshold pattern is switched from the current pattern tothe desired pattern step by step at predetermined time intervals when atleast one of the three or more candidate patterns is present between thecurrent pattern and the desired pattern. In this case, the predeterminedthreshold pattern is prevented from switching from the current patternto the desired pattern at a time while jumping an intermediatepattern(s). Therefore, even if the predetermined threshold patterngreatly changes due to, for example, an abnormality or a defect in thesecond sensor, the above arrangement can avoid a situation that the airbag device is suddenly made much easier to activate, or a situation thatthe air bag device is suddenly made much difficult to activate.Consequently, the air bag device is prevented from being activated bymistake. Thus, according to the invention, the air bag device can beactivated in an appropriate situation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] The foregoing and/or further objects, features and advantages ofthe invention will become more apparent from the following descriptionof preferred embodiments with reference to the accompanying drawings, inwhich like numerals are used to represent like elements and wherein:

[0012]FIG. 1 is a view showing the arrangement of an air bag system thatemploys an activation control apparatus according to a preferredembodiment of the invention;

[0013]FIG. 2 is a graph indicating the relationship between velocity Vnand calculated value f(Gf) that is plotted for each predetermined timeunder certain conditions;

[0014]FIG. 3 is a graph showing variation patterns of variations ofthreshold value SH, which patterns serve as determination maps, whichare plotted in relation to the calculated value f(Gf) and the velocityVn;

[0015]FIG. 4 is a graph useful for explaining the manner of settingthreshold variation patterns according to the preferred embodiment;

[0016]FIG. 5A is a graph indicating changes in a signal level ofsatellite sensors, and FIG. 5B is a graph useful for explaining themanner of switching the threshold variation pattern according to thepreferred embodiment;

[0017]FIG. 6 is a graph representing a situation that is realized whenthe threshold variation pattern is switched in the manner as shown inFIG. 5B; and

[0018]FIG. 7 is a flowchart of an example of a control routine that isexecuted upon switching of the threshold variation pattern.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENT

[0019]FIG. 1 shows the arrangement of an air bag system that employs anactivation control apparatus according to a preferred embodiment of theinvention. The system of this embodiment includes an electronic controlunit (hereinafter referred to as “ECU”) 12 that is installed on avehicle 10, such as an automobile. The air bag system and its activationcontrol apparatus are controlled by the ECU 12.

[0020] The activation control apparatus of this embodiment includes afloor sensor 14 and a pair of satellite sensors 16, 18 that areinstalled on the vehicle 10. The floor sensor 14 is disposed in thevicinity of a floor tunnel located in a central portion of the vehiclebody, while the satellite sensors 16, 18 are disposed on right and leftside members, respectively, in a front portion of the vehicle body. Eachof the floor sensor 14 and satellite sensors 16, 18 consists of anelectronic deceleration sensor that generates a signal indicative of themagnitude of an impact that is applied in a running direction of thevehicle to a relevant portion of the vehicle body at which the sensor islocated. More specifically, the output signal of each sensor representsa deceleration that is experienced by the relevant portion of thevehicle body. Each of the floor sensor 14 and satellite sensors 16, 18also has a self-diagnosis function, and is adapted to generate a certainsignal to the outside when the sensor 14, 16, 18 detects an abnormalityor a defect in itself.

[0021] The ECU 12 includes an input/output circuit 20, a centralprocessing unit (hereinafter referred to as “CPU”) 22, a read-onlymemory (hereinafter referred to as “ROM”) 24, a random access memory(hereinafter referred to as “RAM”) 26 used as a work area, and abidirectional bus 28 through which the components 20, 22, 24, 26 areconnected to each other. The ROM 24 stores in advance various processingprograms and tables needed for operations or calculations.

[0022] The floor sensor 14 and satellite sensors 16, 18 as describedabove are connected to the input/output circuit 20 of the ECU 12. Theoutput signal of the floor sensor 14 and the output signals of thesatellite sensors 16, 18 are supplied to the input/output circuit 20,and are stored in the RAM 26 as needed in accordance with a commandreceived from the CPU 22. The ECU 12 determines a deceleration Gf of thecentral portion of the vehicle body, based on the output signal of thefloor sensor 14. Also, the ECU 12 determines decelerations G_(SL),G_(SR) of the front left portion and front right portion, respectively,of the vehicle body, based on the output signals of the satellitesensors 16, 18. Furthermore, the ECU 12 determines whether each sensoris in an abnormal operating condition, based on an output signal that isgenerated in accordance with the result of self diagnosis by the sensor.

[0023] The system of FIG. 1 further includes an air bag device 30 thatis installed on the vehicle 10 and is activated when appropriate so asto protect an occupant in a passenger compartment of the vehicle 10. Theair bag device 30 includes a drive circuit 32, an inflator 34, and anair bag 36. The inflator 34 incorporates a firing device 38 connected tothe drive circuit 32, and a gas generator (not shown) that generates alarge quantity of gas when it is heated by the firing device 38. The gasgenerated by the inflator 34 is used for deploying or inflating the airbag 36. The air bag 36 is installed in position so that the deployed airbag is located between the occupant of the vehicle 10 and a component(such as a steering wheel) installed on the vehicle.

[0024] The drive circuit 32 of the air bag device 30 is connected to theinput/output circuit of the ECU 12. When a drive signal is supplied fromthe input/output circuit 20 to the drive circuit 32, the air bag device30 is activated, and deployment of the air bag 36 is initiated. The CPU22 of the ECU 12 includes an activation control unit 40 and a thresholdsetting unit 42. The activation control unit 40 of the CPU 22 calculatesa parameter based on a deceleration Gf detected by means of the floorsensor 14, according to a processing program stored in the ROM 24, anddetermines whether the parameter thus obtained exceeds a predeterminedthreshold value. The activation control unit 40 then controls supply ofa drive signal from the input/output circuit 20 to the drive circuit 32of the air bag device 30, based on the result of the above determinationas to whether the parameter exceeds the predetermined value. Thethreshold setting unit 42 suitably sets the above-indicatedpredetermined threshold value to be used by the activation control unit40, based on decelerations G_(SL), G_(SR) determined based on the outputsignals of the satellite sensors 16, 18, respectively.

[0025] Next, a control operation performed by the CPU 22 of the presentembodiment will be described in detail.

[0026] In this embodiment, the activation control unit 40 obtains acalculated value f(Gf) and a velocity Vn by performing predeterminedoperations on a deceleration Gf determined based on the output signal ofthe floor sensor 14. More specifically, the velocity Vn is obtained byintegrating the deceleration Gf with respect to time. Namely, since anobject (e.g;, an occupant) in a passenger compartment of the vehicle isaccelerated forward relative to the vehicle body due to inertial forcewhen the vehicle 10 undergoes a deceleration Gf during running, thevelocity Vn of the object relative to the vehicle can be obtained byintegrating the deceleration Gf with respect to time. The calculatedvalue f(Gf) may be equal to the deceleration Gf itself, or may beobtained by integrating the deceleration Gf with respect to unit time.FIG. 2 is a graph indicating the relationship between the calculatedvalue f(Gf) and the velocity Vn, which relationship is plotted for eachpredetermined time under certain conditions. After obtaining thecalculated value f(Gf) and the velocity Vn, the activation control unit40 compares a value determined from the relationship between thesevalues f(Gf) and Vn, with a threshold value SH that is set by thethreshold setting unit 42 and provides a determination map as describedlater.

[0027]FIG. 3 indicates patterns of variations in the threshold value SH(hereinafter referred to as “threshold variation patters”), whichfunction as determination maps of threshold values plotted against thecalculated value f(Gf) and the velocity Vn. In FIG. 3, five patterns,namely, Hi MAP, Lo1 MAP, Lo2 MAP, Lo3 MAP and FAIL-SAFE MAP, areprovided as threshold variation patterns. In this embodiment, the Hi MAPis a reference map based on which the threshold value SH is normallydetermined, and the FAIL-SAFE MAP overlaps Lo1 MAP. Referring next toFIG. 4, a method of setting a threshold variation pattern according tothis embodiment will be explained.

[0028] In the present embodiment, the threshold setting unit 42 storesin advance the threshold variation patterns that were empiricallydetermined in relation to the calculated value f(Gf) and the velocity Vnas shown in FIG. 3. These threshold variation patterns representboundaries between a region in which the air bag device 30 needs to beactivated in response to an impact applied to the vehicle 10, and aregion in which the air bag device 30 does not need to be activated.

[0029] Since the possibility of collision of the vehicle 10 is higher asan impact acting on a front portion of the vehicle body is greater, itis appropriate to select a suitable one of the above threshold variationpatterns upon receipt of a great impact, so as to make the air bagdevice 30 more likely to be activated. In this embodiment, therefore,the threshold setting unit 42 selects a threshold variation pattern fromthe above five patterns, so that the threshold value SH determinedaccording to the pattern decreases with an increase in the decelerationsG_(SL), G_(SR) determined based on the output signals of the satellitesensors 16, 18. More specifically described referring to FIG. 4, the HiMAP is selected as the threshold variation pattern when a larger one ofthe decelerations G_(SL), G_(SR) is smaller than a first predeterminedvalue G_(S1). The larger one of the decelerations G_(SL), G_(SR) will bedenoted as “deceleration G_(S)”. The Lo1 MAP is selected when thedeceleration G_(S) is equal to or larger than the first predeterminedvalue G_(S1) but is smaller than a second predetermined value G_(S2),and the Lo2 MAP is selected when the deceleration G_(S) is equal to orlarger than the second predetermined value G_(S2) but is smaller than athird predetermined value G_(S3). The Lo3 MAP is selected when thedeceleration G_(S) is equal to or larger than the third predeterminedvalue G_(S3).

[0030] In the activation control apparatus constructed as describedabove, the activation control unit 40 compares the value determined inrelation to the calculated value f(Gf and the velocity Vn, with thethreshold value SH on the threshold variation pattern that is currentlyselected and set by the threshold setting unit 40. If the value definedby the calculated value f(Go and the velocity Vn is greater than thethreshold value SH, a drive signal is supplied from the input/outputcircuit 20 to the drive circuit 32 of the air bag device 30. In thiscase, the air bag device 30 is activated, to initiate deployment of theair bag 36.

[0031] According to the present embodiment, therefore, the activationcontrol apparatus is able to perform suitable activation controldepending upon the type of collision of the vehicle 10, such as ahead-on collision, offset collision or an oblique impact, by changing athreshold value for activating the air bag device 30 depending upon animpact applied to a front portion of the vehicle body. Morespecifically, the air bag device 30 is more likely to be activated asthe magnitude of an impact applied to the vehicle front portionincreases. This make it possible to activate the air bag device 30 at anappropriate time in an appropriate situation.

[0032] In some cases, even if only a small impact is applied to a frontportion of the vehicle body, the decelerations G_(SL), G_(SR) determinedbased on the output signals of the satellite sensors 16, 18 may be largebecause of, for example, an abnormality in the satellite sensors 16, 18,or an abnormality in communications between the satellite sensors 16, 18and the ECU 12, or the Like. In such cases, the deceleration G_(S),which is the larger one of the decelerations G_(SL), G_(SR), may changeat a time from a value that is smaller than the first predeterminedvalue G_(S1), up to a value that is larger than the third predeterminedvalue G_(S3). In this case, the threshold variation pattern switches ata time from the Hi MAP to the Lo3 MAP, to make the air bag 30 much morelikely to be activated, which may result in erroneous deployment of theair bag 36.

[0033] In the system of the present embodiment, even when thedecelerations G_(SL), G_(SR) determined based on the output signals ofthe satellite sensors 16, 18 change by a great degree, the thresholdvariation pattern does not switch at a time from the currently selectedone to a desired pattern, but may be changed step by step, as describedbelow with reference to FIGS. 5A, 5B, 6, and 7.

[0034] Referring to FIG. 5A and FIG. 5B, an operation to switch thethreshold variation pattern according to this embodiment will bedescribed. FIG. 5A shows an example of changes with time in thedeceleration G_(S) (i.e., the larger one of the decelerations G_(SL),G_(SR)) determined based on the output signals of the satellite sensors16, 18. FIG. 5B shows an example of changes in the threshold variationpattern with time in the situation as indicated in FIG. 5A In thisembodiment, the decelerations G_(SL), G_(SR) are detected atpredetermined intervals of sampling time T1 (e.g., 0.5 ms).

[0035] As shown in FIG. 5A and FIG. 5B, the deceleration G_(S)determined based on the output signals of the satellite sensors 16, 18is smaller than the first predetermined value G_(S1) at a point of time(t0-T1), and therefore the threshold variation pattern is set to the HiMAP. If the deceleration G_(S) increases and reaches the thirdpredetermined value G_(S3) at a point of time to in this condition, thethreshold variation pattern is initially set to the Lo1 MAP at the pointof time t1. The threshold variation pattern then switches from the Lo1MAP to the Lo2 MAP upon a lapse of the sampling time T1, namely, at apoint of time t1 as indicated in FIG. 5B. Subsequently, the thresholdvariation pattern switches from the Lo2 MAP to the Lo3 MAP upon a lapseof the sampling time T1, namely, at a point of time t2 as indicated inFIG. 5B. In this operation, the threshold variation pattern does notswitch at a time from the currently selected one to a desired pattern,but may be changed step by step.

[0036]FIG. 6 shows changes in the threshold value in relation to thecalculated value f(Go) and the velocity Vn, when the threshold variationpattern switches with time in the manner as shown in FIG. 5B. In FIG. 6,switching of the threshold variation pattern (i.e., changes in thethreshold value) according to this embodiment of the invention isrepresented by a thick solid line, and switching of the thresholdvariation pattern under conventional activation control is representedby a thick broken line.

[0037] In the above-mentioned conventional arrangement in which thethreshold variation pattern switches to a desired pattern at a time, ifthe velocity Vn is equal to Vn1 at a point of time when conditions forswitching the threshold variation pattern from, for example, the Hi MAPto the Lo3 MAP are established, the threshold variation pattern switchesat a time from the Hi MAP to the Lo3 MAP as indicated by the thickbroken line. In this arrangement, the threshold value on the Lo3 MAPobtained when the velocity Vn is equal to Vn1 is compared with thecalculated value f(Gf), and the comparison between these values ishereinafter made with reference to the Lo3 MAP. As a result, the air bagdevice 30 is activated when the calculated value f(Gf) corresponding toa certain velocity Vn exceeds the threshold value on the Lo3 MAP whichcorresponds to the same velocity Vn.

[0038] With the arrangement of this embodiment in which the thresholdvariation pattern switches step by step at predetermined time intervalsT1, on the other hand, the threshold variation pattern is initiallychanged from the Hi MAP only to the Lo1 MAP as indicated by the thick,solid line in FIG. 6. Then, if the velocity Vn is equal to Vn2 when apredetermined time T1 elapses, the threshold variation pattern switchesfrom the Lo1 MAP to the Lo2 MAP. Subsequently, if the velocity Vn isequal to Vn3 upon a lapse of the predetermined time T1, the thresholdvariation pattern switches from the Lo2 MAP to the Lo3 MAP. In thiscase, the threshold value on the Lo1 MAP is compared with the calculatedvalue f(Gf) when the velocity Vn is equal to Vn1, and the thresholdvalue on the Lo2 MAP is compared with the calculated value f(Gf) whenthe velocity Vn is equal to Vn2. When the velocity Vn is equal to Vn3,the threshold value on the Lo3 MAP corresponding to this velocity Vn3 iscompared with the calculated value f(Gf) corresponding to the samevelocity Vn3.

[0039] According to the instant embodiment as described above, even in asituation where the threshold variation pattern changes greatly due toan abnormality in the satellite sensors 16, 18 or an abnormality incommunications between the sensors 16, 18 and the ECU 12, the air bagdevice 30 is prevented from being easily activated, and erroneousdeployment of the air bag 36 can be thus avoided. Thus, according tothis embodiment, the air bag device 30 can be activated at anappropriate time in an appropriate situation.

[0040]FIG. 7 is a flowchart showing an example of a control routine thatis executed when the ECU 12 switches the threshold variation patternaccording to the present embodiment. The routine as shown in FIG. 7 isstarted each time one cycle of the routine is finished. Once the routineof FIG. 7 is started, step 100 is first executed.

[0041] In step 100, it is determined whether any one of the Lo1 MAP tothe Lo3 MAP is being requested as a desired threshold variation pattern,based on the decelerations G_(SL), G_(SR) determined based on the outputsignals of the satellite sensors 16, 18. Step 100 is repeatedly executeduntil the above condition is satisfied. If it is determined in step 100that any one of the Lo1 MAP to the Lo3 MAP is being requested as thedesired threshold variation pattern, the control process proceeds tostep 102.

[0042] In step 102, it is determined whether the Lo1 MAP is requested inthe above step 100. If an affirmative decision (YES) is obtained in step102, the Lo1 MAP to which the threshold variation pattern switches fromthe Hi MAP only by one step is selected and set as a desired thresholdvariation pattern. In this case, the control process proceeds to step104 in which the threshold variation pattern is switched from the Hi MAPto the Lo1 MAP. After execution of step 104, the value determined inrelation to the calculated value f(Gf) and the velocity Vn is comparedwith the threshold value on the Lo1 MAP. Upon completion of theoperation of step 104, the current cycle of the routine is terminated.

[0043] If a negative decision (NO) is obtained in step 102, on the otherhand, the Lo2 MAP or Lo3 MAP to which the threshold variation patternswitches from the Hi MAP while skipping the Lo1 MAP (and the Lo2 MAP) isselected and set as a desired threshold variation pattern. In this case,the control process proceeds to step 106 in which the thresholdvariation pattern is switched from the Hi MAP to the Lo1 MAP as in theabove-indicated step 104. Thereafter, the value determined in relationto the calculated value f(Gf) and the velocity Vn is compared with thethreshold value on the Lo1 MAP.

[0044] After execution of step 106, step 108 is executed to determinewhether a sampling time T1 has elapsed. Step 108 is repeatedly executeduntil the ECU 12 determines that the sampling time T1 has elapsed. If itis determined that the sampling time T1 has elapsed, the control processproceeds to step 110.

[0045] In step 110, it is determined whether there is any abnormality ordefect in the satellite sensors 16, 18, based on the results of selfdiagnosis of the satellite sensors 16, 18. If step 110 determines thatthere is no abnormality or defect in the satellite sensors 16, 18, thecontrol process proceeds to step 112.

[0046] In step 112, the threshold variation pattern is switched from theLo1 MAP to the Lo2 MAP. After execution of step 112, the valuedetermined in relation to the calculated value f(Gf) and the velocity Vnis compared with the threshold value on the Lo2 map.

[0047] In step 114 following step 112, it is determined whether the Lo2map was requested in the above step 100. If an affirmative decision(YES) is obtained in step 114, no further switching of the thresholdvariation pattern is required. In this case, therefore, the currentcycle of this routine is terminated. If a negative decision (NO) isobtained in step 114, on the other hand, it may be determined that theLo3 MAP was requested in the above step 100. In this case, therefore,the control process proceeds to step 116.

[0048] In step 116 following step 114, it is determined whether thesampling time T1 has elapsed since the operation of step 112 wasperformed. Step 116 is repeatedly executed until the ECU 12 determinesthat the sampling time T1 has elapsed. If an affirmative decision (YES)is obtained in step 116, the control process then proceeds to step 118.

[0049] In step 118, it is determined whether there is any abnormality ordefect in the satellite sensors 16, 18, on the basis of the results ofself diagnosis of these sensors 16, 18, in the same manner as in step110. If it is determined in step 118 that there is no abnormality ordefect in the satellite sensors 16, 18, the control process proceeds tostep 120.

[0050] In step 120, the threshold variation pattern is switched from theLo2 MAP to the Lo3 MAP. After execution of step 120, the valuedetermined in relation to the calculated value f(Gf) and the velocity Vnis now compared with the threshold value on the Lo3 MAP. Upon completionof the operation of step 120, the current cycle of this routine isterminated.

[0051] If it is determined in step 110 or step 118 that there is anabnormality or a defect in the satellite sensors 16, 18, it will not beappropriate to switch the threshold variation pattern to the Lo2 or Lo3map requested in step 100, but will be appropriate to switch the patternto a predetermined FAIL-SAFE MAP. In this case, therefore, the controlprocess proceeds to step 122.

[0052] In step 112, the threshold variation pattern is switched to theFAIL-SAFE MAP. After execution of step 122, the value determined inrelation to the calculated value f(GI) and the velocity Vn is comparedwith the threshold value on the FAIL-SAFE MAP. Upon completion of theoperation of step 122, the current cycle of this routine is terminated.

[0053] According to the process as described above, when the Lo2 MAP orthe Lo3 MAP is newly selected while the Hi MAP as a reference map iscurrently established, and is set as a new threshold variation patternwith the Lo1 MAP (and the Lo2 MAP when the Lo3 is selected) beingskipped, switching from the Hi MAP to the Lo2 MAP or Lo3 MAP is effectedstep by step at predetermined intervals of sampling time T1. Namely, thethreshold variation pattern is initially switched from the Hi MAP to theLo1 MAP, and is then switched to the Lo2 MAP and the Lo3 MAP in thisorder. In this case, since the threshold variation pattern does notswitch to the selected or desired Lo2 MAP or Lo3 MAP at a time, thelikelihood that the air bag device 30 will be activated (i.e., the airbag 36 is deployed) can be advantageously reduced. According to theembodiment, therefore, even when there arises an abnormality in thesatellite sensors 16, 18, or an abnormality in communications betweenthe satellite sensors 16, 18 and the ECU 12, the air bag 36 is preventedfrom being deployed by mistake. This arrangement makes it possible toactivate the air bag device 30 in an appropriate situation In thecontrol process as described above, if an abnormality in the satellitesensors 16, 18 is detected based on the output signals of the sensors16, 18 indicating the results of self diagnosis thereof, during theprocess in which the threshold variation pattern switches from the HiMAP to the Lo2 MAP or to the Lo3 MAP, the switching operation isterminated, and the threshold variation pattern is set to the FAIL-SAFEMAP. In the present embodiment, therefore, when an abnormality arises inthe satellite sensors 16, 18, the threshold value is prevented frombeing set to an undesirably small value, and the likelihood that the airbag device 30 will be activated can be advantageously reduced. Thus, theair bag device 30 can be activated in an appropriate situation even whenan abnormality arises in the satellite sensors 16, 18.

[0054] While the invention has been described with reference topreferred embodiments thereof, it is to be understood that the inventionis not limited to the preferred embodiments or constructions. To thecontrary, the invention is intended to cover various modifications andequivalent arrangements. In addition, while the various elements of thepreferred embodiments are shown in various combinations andconfigurations, which are exemplary, other combinations andconfigurations, including more, less or only a single element, are alsowithin the scope of the invention.

[0055] In the illustrated embodiment, when the decelerations G_(SL),G_(SR) determined based on the output signals of the satellite sensors16, 18 are changed by great degrees, the threshold variation pattern isswitched step by step each time the sampling time T1 elapses. Theinvention, however, is not limited to this arrangement, provided thatthe threshold variation pattern is switched step by step atpredetermined time intervals.

[0056] While the FAIL-SAFE MAP to which the threshold variation patternis set when an abnormality or defect arises in the satellite sensors 16,18 overlaps the Lo1 MAP in the illustrated embodiment, the FAIL-SAFE MAPmay overlap the Lo2 MAP or the Lo3 MAP. Alternatively, the FAIL-SAFE MAPmay be an independently prepared map that does not overlap any of theLo1 MAP, Lo2 MAP and the Lo3 MAP.

[0057] In the illustrated embodiment, the threshold variation pattern isset to a selected one of the Hi MAP, the Lo1 MAP, the Lo2 MAP and theLo3 MAP. The invention, however, is not limited to this arrangement, butmay be applied to any arrangement in which the threshold variationpattern is set to a selected one of at least three maps.

[0058] Furthermore, in the illustrated embodiment, the thresholdvariation pattern is switched step by step from the Hi MAP as areference map to the desired map, e.g., 102 MAP or the Lo3 MAP, withoutskipping the Lo1 MAP or the Lo2 MAP present between the Hi MAP and thedesired map. The invention, however, is not limited to this arrangement.Namely, the invention may be applied to an arrangement in which thethreshold variation pattern is switched from the Lo2 MAP or the Lo3 MAPto the Hi MAP.

1. An activation control apparatus of an air bag system of a motorvehicle, comprising: a first sensor that is disposed at a predeterminedposition within a vehicle body, and is operable to generate an outputsignal indicative of an impact applied to the vehicle; activationcontrol means for activating an air bag device when a parameterdetermined based on the output signal of the first sensor exceeds apredetermined threshold value; a second sensor that is disposed in thevehicle body frontwardly of the predetermined position of the firstsensor, and is operable to generate an output signal indicative of animpact applied to the vehicle; threshold setting means for setting thepredetermined threshold value to a first value selected from at leastthree candidate values, based on the output signal of the second sensor;and threshold switching means for changing the predetermined thresholdvalue from the first value to a second value also selected from the atleast three candidate values, by performing two or more switchingoperations to switch the predetermined threshold value step by step atpredetermined time intervals when at least one of the at least threecandidate values is present between the first value and the secondvalue.
 2. An activation control apparatus according to claim 1, whereinthe threshold switching means switches the predetermined threshold valuefrom one of the at least three candidate values to the next lower onethereof in each of the two or more switching operations.
 3. Anactivation control apparatus according to claim 1 or claim 2, whereinthe output signal of the second sensor is detected each time a samplingtime elapses, and wherein each of the predetermined time intervals issubstantially equal to the sampling time.
 4. An activation controlapparatus according to any one of claims 1-3, wherein the thresholdsetting means sets the predetermined threshold value to a predeterminedfail-safe value upon occurrence of an abnormality in the air bag system,and threshold switching discontinuing means is provided fordiscontinuing switching of the predetermined threshold value by thethreshold switching means when the threshold setting means sets thepredetermined threshold value to the predetermined fail-safe valueduring the switching operations.
 5. An activation control apparatusaccording to any one of claims 1-4, wherein the output signal of thesecond sensor represents a deceleration as measured at a position atwhich the second sensor is mounted.
 6. An activation control apparatusaccording to any one of claims 1-5, wherein the output signal of thefirst sensor represents a deceleration as measured at the predeterminedposition at which the first sensor is mounted.
 7. An activation controlapparatus according to claim 6, wherein the parameter represents a timeintegral of the deceleration determined based on the output signal ofthe first sensor.
 8. An activation control apparatus according to anyone of claims 1-7, wherein the second sensor comprises two satellitesensors that are located in a front right portion and a front leftportion of the vehicle.
 9. An activation control apparatus of an air bagsystem of a motor vehicle, comprising: a first sensor that is disposedat a predetermined position within a vehicle body, and is operable togenerate an output signal indicative of an impact applied to thevehicle; activation control means for activating an air bag device whena parameter determined based on the output signal of the first sensorexceeds a predetermined threshold pattern; a second sensor that isdisposed in the vehicle body frontwardly of the predetermined positionof the first sensor, and is operable to generate an output signalindicative of an impact applied to the vehicle; threshold patternsetting means for setting the predetermined threshold pattern to a firstpattern selected from at least three candidate patterns, based on theoutput signal of the second sensor; and threshold pattern switchingmeans for changing the predetermined threshold pattern from the firstpattern to a second pattern also selected from the at least threecandidate patterns, by performing two or more switching operations toswitch the predetermined threshold pattern step by step at predeterminedtime intervals when at least one of the at least three candidatepatterns is present between the first pattern and the second pattern.10. An activation control apparatus according to claim 9, wherein thethreshold pattern switching means switches the predetermined thresholdpattern from one of the at least three candidate patterns to the nextlower one thereof in each of the two or more switching operations. 11.An activation control apparatus according to claim 9 or claim 10,wherein the output signal of the second sensor is detected each time asampling time elapses, and wherein each of the predetermined timeintervals is substantially equal to the sampling time.
 12. An activationcontrol apparatus according to any one of claims 9-11, wherein thethreshold pattern setting means sets the predetermined threshold patternto a predetermined fail-safe pattern upon occurrence of an abnormalityin the air bag system, and threshold pattern switching discontinuingmeans is provided for discontinuing switching of the predeterminedthreshold pattern by the threshold pattern switching means when thethreshold pattern setting means sets the predetermined threshold patternto the predetermined fail-safe pattern during the switching operations.13. An activation control apparatus according to any one of claims 9-12,wherein the predetermined fail-safe pattern overlaps at least one of theat least three candidate patterns.
 14. An activation control apparatusaccording to any one of claims 9-13, wherein the output signal of thesecond sensor represents a deceleration as measured at a position atwhich the second sensor is mounted.
 15. An activation control apparatusaccording to any one of claims 9-14, wherein the output signal of thefirst sensor represents a deceleration as measured at the predeterminedposition at which the first sensor is mounted.
 16. An activation controlapparatus according to claim 15, wherein the parameter represents a timeintegral of the deceleration determined based on the output signal ofthe first sensor.
 17. An activation control apparatus according to anyone of claims 9-16, wherein the second sensor comprises two satellitesensors that are located in a front right portion and a front leftportion of the vehicle.
 18. A method of controlling activation of an airbag device in an air bag system of a motor vehicle, the air bag systemincluding a first sensor that is disposed at a predetermined positionwithin a vehicle body, and is operable to generate an output signalindicative of an impact applied to the vehicle, and a second sensor thatis disposed in the vehicle body frontwardly of the predeterminedposition of the first sensor, and is operable to generate an outputsignal indicative of an impact applied to the vehicle, the methodcomprising the steps of: activating the air bag device when a parameterdetermined based on the output signal of the first sensor exceeds apredetermined threshold value; setting the predetermined threshold valueto a first value selected from at least three candidate values, based onthe output signal of the second sensor; and changing the predeterminedthreshold value from the first value to a second value also selectedfrom the at least three candidate values, by performing two or moreswitching operations to switch the predetermined threshold value step bystep at predetermined time intervals when at least one of the at leastthree candidate values is present between the first value and the secondvalue.
 19. A method according to claim 18, wherein the predeterminedthreshold value is switched from one of the at least three candidatevalues to the next lower one thereof in each of the two or moreswitching operations.
 20. A method according to claim 18 or claim 19,wherein the output signal of the second sensor is detected each time asampling time elapses, and wherein each of the predetermined timeintervals is substantially equal to the sampling time.
 21. A methodaccording to any one of claims 18-20, wherein the predeterminedthreshold value is set to a predetermined fail-safe value uponoccurrence of an abnormality in the air bag system, and switching of thepredetermined threshold value is discontinued when the predeterminedthreshold value is set to the predetermined fail-safe value during theswitching operations.
 22. A method according to any one of claims 18-21,wherein the output signal of the second sensor represents a decelerationas measured at a position at which the second sensor is mounted.
 23. Amethod according to any one of claims 18-22, wherein the output signalof the first sensor represents a deceleration as measured at thepredetermined position at which the first sensor is mounted.
 24. Amethod according to claim 23, wherein the parameter represents a timeintegral of the deceleration determined based on the output signal ofthe first sensor.
 25. A method of controlling activation of an air bagdevice in an air bag system of a motor vehicle, the air bag systemincluding a first sensor that is disposed at a predetermined positionwithin a vehicle body, and is operable to generate an output signalindicative of an impact applied to the vehicle, and a second sensor thatis disposed in the vehicle body frontwardly of the predeterminedposition of the first sensor, and is operable to generate an outputsignal indicative of an impact applied to the vehicle, the methodcomprising the steps of: activating the air bag device when a parameterdetermined based on the output signal of the first sensor exceeds apredetermined threshold pattern; setting the predetermined thresholdpattern to a first pattern selected from at least three candidatepatterns, based on the output signal of the second sensor; and changingthe predetermined threshold pattern from the first pattern to a secondpattern also selected from the at least three candidate patterns, byperforming two or more switching operations to switch the predeterminedthreshold pattern step by step at predetermined time intervals when atleast one of the at least three candidate patterns is present betweenthe first pattern and the second pattern.
 26. A method according toclaim 25, wherein the predetermined threshold pattern is switched fromone of the at least three candidate patterns to the next lower onethereof in each of the two or more switching operations.
 27. A methodaccording to claim 25 or claim 26, wherein the output signal of thesecond sensor is detected each time a sampling time elapses, and whereineach of the predetermined time intervals is substantially equal to thesampling time.
 28. A method according to any one of claims 25-27,wherein the predetermined threshold pattern is set to a predeterminedfail-safe pattern upon occurrence of an abnormality in the air bagsystem, and switching of the predetermined threshold pattern isdiscontinued when the predetermined threshold pattern is set to thepredetermined fail-safe pattern during the switching operations.
 29. Amethod according to any one of claims 25-28, wherein the predeterminedfail-safe pattern overlaps at least one of the at least three candidatepatterns.
 30. A method according to any one of claims 25-29, wherein theoutput signal of the second sensor represents a deceleration as measuredat a position at which the second sensor is mounted.
 31. A methodaccording to any one of claims 25-30, wherein the output signal of thefirst sensor represents a deceleration as measured at the predeterminedposition at which the first sensor is mounted.
 32. A method according toclaim 31, wherein the parameter represents a time integral of thedeceleration determined based on the output signal of the first sensor.